In 1733, Peter Collinson, a botanist and cloth merchant, walked with great excitement to the ship docks in London. He picked up two boxes of seeds from an American farmer named John Bartram. With these exotic seeds, Collinson transformed English gardens.

In return, Collinson sent Bartram seeds from England and other countries. Andrea Wulf discusses this great seed exchange and friendship in her book, The Brother Gardeners: Botany, Empire and the Birth of an Obsession. It’s a fascinating read. But Wulf fails to address the unintended ecological consequences of this innocent exchange.

A non-native species is a species that humans have transported outside its native range.

Many species were transported intentionally, like those exchanged between Collinson, Bartram and other botanists who traveled the world. White clover (Trifolium repens) was commonly added to lawn mixes; Japanese honeysuckle (Lonicera japonica) was introduced as an ornamental; multiflora rose (Rosa multiflora) and sericea lespedeza (Lespedeza cuneata) were introduced for erosion control and wildlife habitat.

Other non-natives were moved accidentally, like chestnut blight, which arrived in Japanese nursery stock. Chestnut blight decimated the once dominant chestnut (Castanea dentata) and the Ozark chinquapin (Castanea ozarkensis).

A non-native invasive species is one that spreads beyond its place of introduction, often taking over an area and preventing the growth of native species.

But why does it matter? The book Bringing Nature Home, by Douglas Tallamy answers this question. As Tallamy says, we need to understand plants’ roles in their ecosystems. Plants do not grow in isolation from insects, birds, and other creatures – they form relationships that are essential to each other’s existence.

Monarch caterpillars can only eat milkweeds. Many other pollinator species require native plants for their caterpillars. Photo by Vicki Deloach.

Insects gather pollen and nectar and, in return, plants are pollinated. But the relationship is not that simple. Pollinators also depend on plants to feed their larval stage (i.e. caterpillars). Plants develop new toxins to deter caterpillars from eating them. Insects develop ways to deal with those toxins. For example, monarch butterflies developed the ability to ingest milkweed toxins. These relationships create a system of checks and balances to ensure that population sizes of all plants and animals are kept at a level that can be supported by the ecosystem. And this often maximizes diversity.

When a plant is transferred from another part of the world, the ecological constraints that kept this plant in check are gone and so are the links that made it a contributing member of its ecosystem.

Non-native plants have no evolutionary history with our native insects and as a result, insects typically don’t eat them. So we see kudzu (Pueraria montana) covering hillsides with nary a nibble from its perfect leaves. All of that habitat that once provided space for native plants and food for native insects is now gone.

Plants are the base of the food chain, and as they decline in diversity and population size, so do the insects, birds, reptiles, and mammals that depend on those primary producers.

The need to rid our communities of exotic plants is clearer than ever. Non-native plants take up space in our yards, space that could be filled native plants that benefit ecosystems.

Autumn-olive is native to China, Japan and Korea and was introduced in 1830 to provide wildlife habitat and erosion control. It is a deciduous shrub with silvery scales that give it a grayish green appearance. Its tart, edible red berries are widely dispersed by birds, enabling its widespread invasion. Its nitrogen-fixing root nodules allow it to invade even the most infertile soils. Autumn-olive can be removed by pulling small seedlings, but if it is too large to pull, herbicide is needed – burning and cutting just make it mad! Native alternatives include serviceberry (Amelanchier arborea), dogwoods (Cornus drummondii, C. racemosa or C. obliqua) and blackhaw (Viburnum prunifolium).

Chinese privet is another lush-smelling non-native invasive shrub. It hails from China, Taiwan and Vietnam. It can reach twenty feet tall and has opposite elliptical leaves and small white fragrant flowers at the tips of branches. It is especially prone to invading high quality natural communities such as glades. Alternatives to privet include flowering dogwood (Cornus florida), redbud (Cercis canadensis), serviceberry (Amelanchier arborea), and native azaleas.

Japanese honeysuckle is a woody vine that was introduced from East Asia in the 1800s as an ornamental and has been widely planted over the last century. While its luscious smell is intoxicating, its ability to invade native forests and exclude native vegetation is equally unmistakable. It has opposite leaves and white flowers that eventually turn yellow. There are several lovely native honeysuckles including yellow honeysuckle (Lonicera flava), which has stunning yellow flowers and trumpet honeysuckle (Lonicera sempervirens) has beautiful red tubular flowers that hummingbirds love.